Technical Field
The present invention relates to electrochemical fuel cells and, in particular, to flow field channel features for flow field plates.
Description of the Related Art
Fuel cells convert fuel and oxidant to electricity and reaction product. Proton exchange membrane fuel cells employ a membrane electrode assembly (“MEA”) having a proton exchange membrane (“PEM”) (also known as an ion-exchange membrane) interposed between an anode electrode and a cathode electrode. The anode electrode typically includes an electrocatalyst and an ionomer, or a mixture of electrocatalyst, ionomer and binder. The presence of ionomer in the electrocatalyst layer effectively increases the electrochemically active surface area of the electrocatalyst, which requires an ionically conductive pathway to the cathode electrocatalyst to generate electric current. The cathode electrode may similarly include electrocatalyst and binder and/or ionomer. Typically, the electrocatalyst used in the anode and the cathode is platinum or platinum alloy. Each electrode may further include a microporous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a gas diffusion layer (GDL). The anode and cathode electrodes may be bonded or sealed to the PEM to form a single integral MEA unit.
The membrane electrode assembly is typically interposed between two electrically conductive flow field plates or separator plates. These flow field plates act as current collectors, provide support for the electrodes, and provide flow fields for the supply of reactants, such as fuel and oxidant, and removal of excess reactants and products that are formed during operation, such as product water. In some cases, the bipolar flow field plate is formed by joining two flow field plates together, namely, an anode flow field plate and a cathode flow field plate, so that an anode flow field is formed on one surface of the bipolar flow field plate, a cathode flow field is formed on an opposing surface of the bipolar flow field plate, and a coolant flow field is formed between the anode flow field plate and the cathode flow field plate. In other cases, the bipolar flow field plate may be a single plate that has an anode flow field on one surface and a cathode flow field on an opposing surface. The flow fields typically contain a plurality of landings, between which flow field channels are formed, and contact the electrodes of the MEA when assembled into a fuel cell. FIGS. 1-4 (prior art) collectively illustrate a typical design of a conventional MEA 5, with electrodes 1,3 sandwiching a proton exchange membrane 2 therebetween (FIG. 1); an electrochemical cell 10 comprising an MEA 5 between flow field plates 11, 12 (FIG. 2); a stack 50 of electrochemical cells 10 (FIG. 3); and stack 50 compressed between endplates 17, 18 (FIG. 4). FIGS. 1-4 each also illustrate manifolds 30 for delivering and removing reactants and products to and from the fuel cells during operation.
During fuel cell operation, a primary load is drawn from the fuel cell. At the anode electrode, fuel (typically in the form of hydrogen gas) reacts at the anode electrocatalyst in the presence of the PEM to form hydrogen ions and electrons. At the cathode electrode, oxidant (typically oxygen in air) reacts with the hydrogen ions, which pass through the PEM, in the presence of the cathode electrocatalyst to form water. The PEM also serves to isolate the fuel stream from the oxidant stream while facilitating the migration of the hydrogen ions from the anode to the cathode. The electrons pass through an external circuit, creating a flow of electricity to sustain the primary load.
In practice, fuel cells need to be robust to varying operating conditions, particularly to conditions in which liquid water accumulates in the flow field channels. When liquid water is present, some fuel cells in the fuel cell stack may exhibit a significantly higher flow resistance (i.e., less gas flow through the flow channel for a given pressure drop) than other fuel cells in the fuel cell stack. As a result, the lower flow resistance fuel cells will experience more gas flow than the higher flow resistance fuel cells, causing the higher flow resistance fuel cells to become starved of reactants and leading to a decrease in their voltage. Such flow resistance may vary from cell to cell and may randomly occur over time due to the presence of liquid water in the flow channel causing instabilities with regard to voltage and/or pressure.
A number of techniques have been proposed to remove liquid water in the flow channels or to keep liquid water from forming. In one example, the fuel cell stack can be operated at higher temperatures to reduce or prevent liquid water from forming. However, the maximum operating temperature is typically limited due to degradation of the components, such as degradation of the proton exchange membrane and seal materials, as well as corrosion of the carbonaceous components. In other examples, the flow channels can be designed to exhibit a higher pressure drop for a given flow rate and/or the reactants can be supplied at a higher flow rate (e.g., periodically purging) so that liquid water can be removed more easily. However, these techniques require relatively expensive fuel cell system design, more robust MEA components, and/or increased system parasitic losses.
Due to the shortcomings with modifying the operating conditions to remove liquid water, it has been suggested to modify the flow channel features such that water is wicked away from the electrodes. For example, U.S. Pat. No. 6,649,297 discloses a bipolar plate for a fuel cell comprising, on at least one of its faces, a groove able to form a gas distribution channel with the surface of an adjacent electrode, wherein the gas distribution channel has a shape or geometry such that the liquid of the biphasic flow flowing in the channel may be moved away from the electrode interface. In one preferred geometry, the channels have a transverse section in the shape of an isosceles trapezium, the sides of which (other than the bases) are equal and the small base of which is defined by the surface of the electrode. Stated differently, and as shown in FIG. 5 (prior art), the angles near the electrode (angle φ) both have a larger value than the two opposite angles (angle β). However, such a distribution channel geometry is difficult to manufacture in high volumes using low-cost plate molding, embossing, or machining techniques.
In another example, U.S. Pat. No. 7,087,337 describes an assembly for a fuel cell including an electrically conductive fluid distribution element with a flow field disposed on a surface of the element, wherein the flow field includes a plurality of channels for carrying the gaseous reactants of the fuel cell. The assembly also includes an electrically conductive member disposed at the surface of the element to serve as a gas diffusion media. As illustrated in FIG. 6 (prior art), the channels of the element include a plurality of sidewalls (61,62) formed in various orientations, and the orientations of the sidewalls form a cross-sectional geometry of the channel such that water collection regions are formed at an interface of the electrically conductive fluid distribution element and the electrically conductive member, and at a bottom portion of the channel. When using a ramped sidewall geometry for the channels, the water is drawn into the sharpest corner of the channel. However, the gas diffusion layer is typically hydrophobic, which makes them poor surfaces to form sharp corners for accumulating water. Furthermore, for water to accumulate at the bottom portion of the channel, the depth of the channel needs to be shallow, which is limited by manufacturability and constrains the flow channel dimensions and design flexibility.
Accordingly, there remains a need for improved techniques to remove liquid water in fuel cells. The present invention addresses this need and provides further related advantages.